Question

What happen when bromobenzene is treated with HNO3

Answer 1

One method of nitrating bromobenzene involves mixing HNO3 and sulfuric acid with bromobenzene. The reaction mechanism involves protonation of nitric acid by sulfuric acid (stronger acid) and subsequent dehydration to generate nitronium ion, which is attacked by the benzene ring to form a nitrobromobenzene. Although bromine is a weak ring deactivator, it is orthopara director so the dominant product will be nitrated either in ortho or para direction. The factors that determine whether ortho or para is more favored are steric hindrance and electronegativity. Statistically, ortho carbon should collide with Br2 more often because two carbons are available for ortho position while only one carbon is available for para position, and thus there should be 21 distribution ortho para product if the substitution is random. However, the product distribution shows 37 ortho 62 para (with 1 meta product) so the reaction is not statistically controlled. Such distribution can be explained by steric hindrance since bromine is a big atom, the formation of ortho product will require higher activation energy than that of the para product, which wont be as sterically hindered by the bromine atom. If steric hindrance was the only major factor that determined distribution of products, then of para product should be lower when chlorobrenzne is nitrated since Cl is smaller than Br. However, data analysis shows that even more para product will be formed for chlorobenzene. Thus the reaction is not controlled by sterical hindrance alone. Instead, the difference in electronegativity between Cl and Br could explain this increase in para product formation. Since Cl is more electronegative than Br, the carbon attached to Cl will have more partial positive charge than the carbon attached to the bromine. Therefore, the ortho product from chlorobenzene will create a carbocation near the already partially positive carbon, which is less stable than the intermediate for the para product because positive charges tend to repel each other especially in close proximity

Answer 2

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